1. Field of the Invention
The use of fiber-reinforced composite materials with thermosetting resins is increasing in the aerospace industry. Fibers such as glass, boron, graphite, and carbon are combined with resins such as epoxy, phenolic, polyimide and the like to achieve composites with high strength-to-weight ratios. The drawback of these composites is fabrication costs. Since the basic composite characteristics of strength, thickness, resin content and porosity are established at cure, finding an inexpensive cure control method, that would assure the desired property, has been a difficult task.
When a manufacturer receives a fabric that is impregnated with resin (prepreg), the resin is within an intermediate reaction state called the B-stage. The prepreg is pliable or will soften when warmed. With time and temperature the resin reaction will continue. Molecular weight increases due to polymer formation and cross linking as the resin advances through the B-stage state. At full advancement, the end of the B-stage reaction is reached when vitrification of the elastomeric structure occurs and the resin is no longer plastic when hot. Therefore, the layup and molding of production parts must be accomplished while the prepreg is still within the B-stage and is plastic when heated.
Cure of thermosetting resins is often done in an autoclave. One type of autoclave operating technique is to initially set the pressure at a low or moderate level. The part temperature is then increased, and the resin reaction advances within the B-stage state to a selected degree of polymer formation at which time the autoclave pressure is raised to create desired physical characteristics in the laminate. These characteristics are resin content, thickness, strength, and minimum microporosity. If the proper application of pressure in the autoclave is to produce a certain thickness or other physical property, for example, then that pressure must be applied when the resin has advanced to the proper amount of polymer formation but while it is still plastic and fusible. It is necessary therefore, to develop a method of monitoring and controlling the resin advancement within the B-stage while the resin is still plastic when hot.
As the resin advances in polymerization, the molecular weight increases. Viscosity also increases, and attempts have been made by other investigators to monitor viscosity change, by either dielectric changes or ultrasonic attenuation methods, to determine the pressure application point. But these methods are not without drawbacks, since viscosity is a function of temperature as well as advancement within the B-stage state. This means laminates at different temperatures that have the same viscosity reading could actually be at different states of advancement. Therefore, viscosity measurements would have to be correlated with thermal history to know the actual degree of advancement. This adds to the complexity of using viscosity measurements as an indication of resin advancement. Dielectric change measurements are also impractical when the reinforcing fibers are electrically conductive or when conductive fillers are added to the resin.
Autoclave operational techniques also have severe limitations since quite often a single autoclave cure cycle will be developed for use with all of the existing production parts. In this type of cycle, the temperature-time history and the pressure-time history, including the pressure application point, are preselected. Variation from laminate to laminate occurs in this type of cure even though the same amount of B-stage advancement is added in each autoclave run. This limitation is caused by the laminates coming to the autoclave at different degrees of advancement because of variations in their accumulated time-temperature histories, during the lay-up process.
These limitations indicate the need for a better technique to monitor resin advancement from the start of layup through cure so that consistent desired properties may be achieved.
2. Background art
A review of the prior art reveals the following U.S. patents which relate to technology more or less pertinent to some aspects of the present invention.
U.S. Pat. No. 3,718,721 to Gould, et al. discloses a method for controlling the state of cure in a mold which involves the monitoring of the temperature as a function of time. The state of cure of a predetermined site in the article is computed from the temperature measurements.
U.S. Pat. No. 3,049,410 to Warfield, et al. shows a method of determining the optimum temperatures for bulk curing of resins. It involves determining minimum curing time and determining the effect of temperature on the rate of cure using measurement of changes in electrical resistivity of samples during polymerization.
U.S. Pat. No. 3,413,836 to Nadeau, et al. discloses an apparatus for detecting a change of state in a liquid sample, such as a gel point or freezing point, involving detection of changes in amplitude of oscillatory motion of a liquid sample and a reference fluid within two containers.
U.S. Pat. No. 4,312,212 to Clendenin shows an apparatus for measuring the length of time necessary to heat an epoxy resin prepreg to a tacky condition.
None of the prior art including the above patents relate however specifically to a percent gel parameter for controlling an autoclave nor do they relate to a process which includes such features as (1) determining the initial state of cure of a sample, together with (2) determining the amount of total energy (time and temperature) required to complete gellation, (3) accounting for the resin cure advancement at all stages of processing so that (4) final processing steps (typically autoclave) may be adjusted to achieve pressure application at the optimum point in the processing cycle prior to 100% gellation. None of the prior art teaches a method sensitive enough to achieve uniform product results despite differences in the layup history of various batches.